# Dex: Mandelbrot set

The following is a port of mandelbrot.dx.

The escape time algorithm needs complex numbers, so we import the definition from the complex number example.

```
module complex = import "complex-numbers"
module c64 = complex.c64
type c64 = c64.complex
let update (c: c64) (z: c64) = c64.(c + z * z)
let tol : f64 = 2.0
let inBounds (z: c64) = c64.mag z < tol
```

In Dex, type trickery (the `Fin 1000`

type) is used to implicitly fold across an imaginary 1000-element array. In Futhark, we use an old-fashioned `for`

-loop.

```
let escapeTime (c: c64) =
loop (n, z) = (0, c64.mk 0 0) for _i < 1000 do
(let z' = update c z
in (n + f64.bool (inBounds z'), z')).0
```

A more idiomatic Futhark implementation would use a `while`

-loop (as here).

We finish up with a simple `main`

function that produces a two-dimensional array of floats.

```
let main h w =
2d h w
tabulate_
(\j i ->let x = -2.0 + f64.i64 i * (3/f64.i64 w)
let y = -1 + f64.i64 j * (2/f64.i64 h)
in 1-escapeTime (c64.mk x y) / 1000)
```

`> :img main 200i64 300i64`